British Pat. No. 1,363,075 disclosed that a useful .beta.-lactamase inhibitor could be obtained by the fermentation of certain strains of Streptomyces olivaceus. Until our present invention, it was believed that the material disclosed in British Pat. No. 1,363,075 was substantially pure. However, we have discovered that this is not so and that a minor component of that material can be isolated and has potent antibacterial activity. This new material is designated MM 13902 and it is now believed to be responsible for a part of any antibacterial activity present in the material of British Pat. No. 1,363,075 although it is responsible for only a very minor part of .beta.-lactamase inhibitory activity exhibited by that material. Naturally, nothing in this specification should be construed as claiming material as disclosed in British Pat. No. 1,363,075. Other .beta.-lactamase inhibitors are known to be produced by strains of Streptomyces, for example those disclosed in German Published Patent Application No. 2,340,005, but such known materials have not been demonstrated as having the potent antibacterial activity of the kind possessed by MM 13902.